CMOS power-on reset circuit

ABSTRACT

A CMOS power-on reset circuit has a delay capacitor to provide a predetermined delay period. Charging and discharging of the delay capacitor is controlled by the state of a flipflop circuit. An input comparator monitors a power supply input voltage. An invalid input voltage level immediately changes the reset output signal to the invalid state and discharges the capacitor. Even after the input voltage has recovered to a valid level, recharging the capacitor is delayed until the capacitor has substantially discharged, thereby ensuring at least a predetermined delay period after the last fault condition. The reset output signal is coupled in a feedback configuration so as to lower the threshold voltage when the reset output switches to the valid state, to allow limited power supply sag, for example due to motor start-up, without resetting the circuit. Multiple power supply voltages are continuously monitored in a CMOS integrated configuration by additional input scaling resistor networks and input comparators, all coupled to the common 2-level threshold voltage node.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a divisional of application Ser. No. 07/891,587 filed on Jun. 01, 1992 and now U.S. Pat. No. 5,369,310 issued Nov. 29, 1994.

This invention relates generally to methods and circuits for generating a reset signal useful for resetting electronic circuitry to a predetermined state responsive to power-up of a power supply, or following a dip or glitch in the power supply voltage. More specifically, this invention relates to a power-on reset method and circuits for implementing the method for precision applications in CMOS integrated circuit technology.

BACKGROUND OF THE INVENTION

Electronic circuits, particularly logic circuits including microprocessors, are powered by at least one external power supply. When a power supply is initially turned on, the powered logic circuits must be initialized to a known state. Even after such circuitry has been initialized and is operating properly, the power supply voltage may for some reason dip or glitch unexpectedly, causing the logic circuitry to change state or otherwise operate improperly. It is therefore necessary to continuously monitor the power supply voltage and to reset the logic circuitry following such a glitch, after the power supply voltage reaches a predetermined minimum voltage adequate for proper operation. A reset operation, upon initial power-up or after a power supply glitch, is effected by a power-on reset logic signal. The present invention relates to monitoring power supply voltages and providing such a reset signal.

In the past, external reset circuits comprising discrete components have been utilized to provide a reset signal to an integrated circuit such as a microprocessor. Other types of voltage monitoring circuits have been used to monitor AC power lines. For example, international patent application no. PCT/US90/06301 owned by Voltage Control, Inc. discloses a discrete, analog circuit for monitoring an AC line voltage and disconnecting AC powered user equipment from the AC power line whenever the AC power line voltage is outside a predetermined operating voltage window. Use of discrete components is undesirable for most applications because of their large size and the cost of implementing discrete components.

U.S. Pat. No. 5,109,163 to Benhamida discloses a power-on reset circuit which may be implemented as part of an integrated circuit chip including a microprocessor or other logic circuitry which may require initialization at power-up. U.S. Pat. No. 5,115,146 also shows an integrated power-on circuit. Monolithic power-on reset circuits are commercially available in bipolar technology. Examples of such devices are the TL7702 series of "supply voltage supervisors" made by Texas Instruments and the Fujitsu device number MV 3771.

Prior art power-on circuits provide for a timing capacitor (internal or external) to provide a delay period after a monitored power supply voltage is valid before asserting the reset signal. This assures that the power supply voltage has stabilized. Typically, the reset signal is derived from the capacitor node and is asserted when the capacitor is sufficiently charged. When the power supply voltage drops to an invalid level, a transistor switch discharges the timing capacitor. After the capacitor is sufficiently discharged, the reset signal changes to the invalid state. For example, the Fujitsu part No. MB 3771 employs an NPN transistor arranged to discharge an external timing capacitor (node C_(T)) when the monitored voltage becomes invalid. The reset output signal is derived from that capacitor node so that, after the capacitor is sufficiently discharged, the reset output signal changes state. A similar approach is shown in U.S. Pat. No. 5,109,163 to Benhamida (see FIG. 1, internal capacitor node C). In the prior art, switching the reset signal to the invalid state thus must await discharge of the timing capacitor. Discharging the timing capacitor introduces delay after the power supply voltage has dropped before asserting the reset signal. Such delay can lead to malfunction, particularly in high-speed systems. A need remains for asserting a reset signal to indicate an invalid power supply voltage without delay whenever the power supply voltage drops to an invalid level.

The prior art discrete and bipolar circuits are not amenable to implementation in CMOS technology. Moreover, both the circuits disclosed in the '163 patent (Benhamida) as well as the bipolar monolithic solutions suffer several common shortcomings. First, some prior art circuits employ input comparators with hysteresis. Hysteresis on the input has been found to be of little benefit. This is because, after an initial fault condition occurs, a relatively long delay period ensues during which the input is ignored. Additionally, hysteresis adds to the size and complexity of the comparators and makes it difficult to expand such reset circuits to monitor multiple power supply voltages.

The need remains, therefore, for an improved method of providing a power-on reset signal in a CMOS integrated system, and for circuitry for implementing such a method that is relatively simple and therefore compact in size. It would also be desirable to provide a power-on reset circuit which obviates providing hystersis in the input stage. The need also remains for providing an immediate indication of an invalid power supply voltage level.

Additionally, many integrated circuit systems require multiple power supply voltages. For example, a typical CMOS circuit may require both a +5 VDC supply (VDD) and a +12 VDC supply (VHH). It is therefore desirable to provide for monitoring multiple power supply voltages in a power-on reset circuit while minimizing circuit size and complexity.

SUMMARY OF THE INVENTION

An object of the invention is to provide accurate power supply voltage monitoring in a CMOS integrated circuit.

Another object of the invention is to provide a reset logic signal that immediately changes state in response to an invalid power supply voltage.

A further object is to simultaneously monitor multiple power supply voltages in a simple and compact integrated circuit.

One aspect of the invention is a power-on circuit for generating a binary reset output signal having a valid state and an invalid state. The invalid state of the reset output signal indicates that one or more power supply voltages has fallen below a predetermined minimum level acceptable for normal operation. In other words, a supply voltage has become "invalid". It may have done so only momentarily, i.e in a glitch, or over a longer period. The power-on reset output signal does not become valid until the input power supply voltage has remained continuously above its minimum valid level.

The power-on circuit includes a resistive divider network for receiving and scaling a power supply voltage (PORVDD) to be monitored. A threshold voltage is derived from a reference voltage. A first comparator is employed to compare the scaled power supply voltage to the threshold voltage. The comparator output indicates a valid power supply state when the scaled power supply voltage exceeds the threshold voltage, and indicates an invalid power supply state otherwise.

A delay timer provides a predetermined delay period. Logic circuitry starts the delay timer responsive to the indication of a valid power supply state. However, further logic delays said starting the delay timer until after the delay timer indicates completion of the delay period. This ensures that at least the delay period elapses since the power supply last becoming invalid before changing the reset output signal to the valid state.

Additional logic is coupled to the first comparator for forcing the reset output signal to an invalid state responsive to the indication of an invalid power supply state and without regard to the delay period, thereby immediately forcing the reset signal to the invalid state without delay.

Preferably, the invention is implemented in a CMOS integrated circuit wherein the delay timer includes an RS flipflop. A delay capacitor stores a capacitor voltage. Means are provided for charging the delay capacitor while an intermediate signal, i.e. the flipflop Q output signal, is in a first state and for discharging the delay capacitor while the intermediate signal is in a second state.

A second comparator monitors the delay capacitor voltage and compares it to a predetermined reference voltage to determine when the delay capacitor is substantially discharged. Recharging the capacitor is delayed, even after the power supply voltage is valid, until the capacitor is substantially discharged. The time to recharge the capacitor from that discharged state defines the delay period.

Circuitry for starting the delay timer thus is coupled to the first comparator for setting the intermediate logic signal to the first state to begin charging the delay capacitor responsive to the indication of a valid power supply state; and delaying means includes the second comparator and is coupled to the first comparator for setting the intermediate logic signal (q) to the second state to begin discharging the delay capacitor only when the power supply voltage is invalid and after the delay capacitor is substantially discharged, thereby ensuring completion of the delay period before starting a new delay period.

Importantly, the first (input) comparator is coupled to the flipflop so as to force the flipflop to a predetermined state whenever the input power supply voltage falls below its threshold. The flipflop output in turn forces the output reset signal to its invalid state, thereby indicating an invalid input state without any appreciable delay. The delay capacitor voltage does not affect, and does not delay, switching the reset output to the invalid state--only switching to the valid state.

Multiple power supplies may be monitored by providing a scaling resistor network and a comparator for each additional input.

The foregoing and other objects, features and advantages of the invention will become more readily apparent from the following detailed description of a preferred embodiment which proceeds with reference to the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a CMOS power-on reset circuit according to the present invention.

FIG. 1A is a schematic diagram showing an alternative feedback resistor T-network for use in the circuit of FIG. 1.

FIGS. 2A through 2E are simulation plots illustrating operation of the circuit of FIG. 1 modified so as to employ the alternative feedback resistor network of FIG. 1A.

FIGS. 3A and 3B are expanded plots (×1000) showing detail of selected waveforms of FIG. 2 expanded about a time equal to 600 msec.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Introduction

FIG. 1 is a schematic diagram of a CMOS power-on reset circuit 10 according to the invention. In this embodiment, two power supplies PORVDD and PORVHH are monitored simultaneously. The input voltages PORVDD and PORVHH are scaled down using resistive divider networks. Each of the divided outputs is connected to a corresponding input comparator for comparison to a threshold voltage. The results of those comparisons, together with delay timing described below are used to provide the power-on reset logic signal reset₋₋ h. A low voltage (logic 0) on reset₋₋ h is the valid state. It indicates that the monitored power supply voltages have all remained continuously above their respective valid levels for at least a delay period described below.

Threshold Voltage

The threshold voltage vp is generated internally as follows. A first reference voltage vref is a relatively accurate external (or internal) reference voltage. Preferably, vref is derived such that on power-up of the power-on circuit, vref follows PORVDD until it exceeds the steady state value of vref. vp is derived from vref through R1-R2. Comparator 4 is connected in a positive feedback configuration using the R1-R2 resistor network. The purpose of the positive feedback is to provide hysteresis. The value of vp is determined as follows:

    vp=[R2/(R1+R2)]*vref for vo4=logic 0

    vp=[R2/(R1+R2)]*vref+[R1/(R1+R2)]*vo4 for vo4=logic 1.

Comparator 4 output vo4 goes low at the end of the delay period. This is when reset₋₋ h typically goes low, i.e. to the valid state. Because of summing at the vp node, vp is higher when vo4 is high, and vp drops when vo4 goes low. This lowers the vp threshold voltage.

This arrangement is particularly well suited for applications, such as disk drives, where the power supply will drive a motor. When the reset signal goes valid, the motors go on, causing the power supply voltage to temporarily sag. This can cause the reset signal to switch to the invalid state, shutting off the motors, etc. in an unstable a potentially harmful loop. Lowering the threshold voltage when the reset goes valid avoids this problem.

By providing a threshold voltage itself having hysteresis, the input comparators (comparator 1, comparator 2) need not have hysteresis built into the comparators. This allows for easy expansion of the circuit to simultaneously monitor additional power supply voltages. Each additional power supply to be monitored requires only a suitable resistive divider and a simple comparators. No separate provisions for hysteresis are required, as it is built into the threshold voltage vp. Each resistive divider is designed to scale the corresponding nominal power supply voltage down to the threshold voltage vp.

Input Comparator Circuitry

Input comparators, COMPARATOR 1 and COMPARATOR 2, continuously monitor the applied power supply voltages. The first power supply PORVDD input voltage is divided by a resistive divider network Rd1-Rd2, and the divided output signal vsen1 is input to the inverting input of Comparator 1. Similarly, PORVHH is divided by the Rh1-Rh2 network, and the divided output signal vsen2 is connected to the inverting input of Comparator 2. The non-inverting inputs to Comparators 1 and 2 are coupled to receive the threshold voltage vp. Resistors Rd1-Rd2 and Rh1-Rh2 are ratioed such that when the power supplies PORVDD and PORVHH reach their valid levels, voltages at vsen1 and vsen2 are equal to vp with vo4 high (logic level 1). The input comparator circuitry may be expanded for simultaneously monitoring additional power supplies by providing a similar resistive divider network and input comparator for each power supply.

The output of Comparator 1, vo1 and the output of Comparator 2, vo2 are combined in a NOR logic gate 22. The output of NOR gate 22 is coupled through an inverter 24 to the set input of an RS flipflop 20. (Additional input comparators may be accommodated by expanding the number of inputs of NOR gate 22 as necessary.) Accordingly, when both (or all) power supply voltages are valid, i.e. when all scaled input voltages exceed vp, the output of NOR gate 22 is high (logic 1).

Delay Timing

The delay timing circuitry is described next. The delay timing circuitry includes a delay capacitor 30 and a current source 36 for charging the capacitor 30. Current source 36 is coupled to capacitor 30 through a transistor 34 for charging the capacitor. The capacitor is coupled to ground through a transistor 32 for discharging the capacitor. Transistors 32,34 are complementary so that only one of them is ON at one time. The gates of transistors 34,32 are coupled together to the Q output of flipflop 20 so that when the flipflop output signal (q) goes low (logic 0), transistor 34 turns ON and the delay capacitor begins to charge. Conversely, when q goes high (logic 1), transistor 34 turns OFF and transistor 32 turns ON to discharge the delay capacitor as illustrated in FIG. 3. The state of RS flipflop 20 thus determines whether the delay capacitor is charging or discharging. Delay capacitor 30 remains charged in the normal (valid) steady state.

Comparator 3 continuously monitors the charge of voltage on the delay capacitor 30. A second reference voltage vref2 is derived from the reference voltage vref by a resistive divider network Rr1-Rr2. Comparator 3 has an inverting input coupled to delay capacitor 30 and a non-inverting input coupled to receive the second reference voltage vref2. Accordingly, the output of Comparator 3 vo3 goes high (logic 1) when the delay capacitor voltage falls below vref2. Vref2 is set to a relatively low voltage (about 0.6 volts in an example given below), so that Comparator 3 detects and indicates when the delay capacitor is substantially discharged (see FIG. 3A).

The output of comparator 3 is coupled through a logic AND gate 38 to the reset input of flipflop 20. The second input to AND gate 38 is connected to the output of NOR gate 22. Recall that NOR gate 22 is high (logic 1) when both power supply voltages are valid, The output of Comparator 3 vo3 gates that signal through AND gate 38 to the reset input of flipflop 20. Accordingly, the flipflop can reset Q low, to begin charging the delay capacitor, only when the power supply voltages are valid AND the capacitor is substantially discharged. This is to ensure that at least a full delay period elapses after a most recent power supply glitch before the reset output signal changes to the valid state. The delay period is defined by the time necessary to charge the delay capacitor 30 from a low voltage (i.e. less than vref2) to a higher voltage (vp). This delay time depends on the size of the capacitor, of course, as well as the charging current source 36. It is selected as appropriate for the desired application. For disk drive motor control, for example, a long delay such as 250 msec is desirable. The delay time (td) from when the power supply voltages are valid to the time when reset₋₋ h goes low is determined by: td=(C*vp)/I where I is the current provided by current source 36 and C is the capacitance of capacitor 30.

Comparator 4 and Reset output signal

Comparator 4 serves the dual purposes of holding the reset signal in the invalid state (logic 1) during the delay period, and providing positive feedback from the reset output to impart hysteresis to the threshold voltage vp. Comparator 4 has an inverting input coupled to the delay capacitor 30 and a non-inverting input coupled to the threshold voltage vp. In the normal steady state, capacitor 30 is charged, so that the output of comparator 4 (vo4) is low (logic 0). This allows the Q output signal (flipflop 30) to pass through an OR gate 40 to provide the reset signal.

In the valid steady state, Q is low (logic 0) so reset₋₋ h is low as well. However, when capacitor 30 is discharged and while it is charging, i.e., while its voltage is less than vp, vo4 is high, thereby forcing OR gate 40 and hence the reset signal high, indicating the invalid state. As noted above, when vo4 is high, it also contributes to vp through feedback resistor R2, to provide hysteresis. Consequently, vp is slightly higher during the invalid state.

It should be noted that RS flipflop 20 has its output Q coupled through an OR gate 40. The output of OR gate 40 provides the reset signal useful for resetting or initializing other circuitry within a monolithic CMOS system. Alternatively, or additionally, the reset signal may be brought out to an external pin using appropriate driver or buffer circuitry, details of which are known.

It is noteworthy that the reset signal is not controlled directly from the delay capacitor 30. Rather, the reset signal is isolated from the delay circuit, as will be explained below, in order to avoid delay associated with discharging the delay capacitor, in order to immediately indicate an invalid state of one or more of the input voltages.

Operative Example

In an operative example of the preferred embodiment of the invention, implemented in a CMOS process, components and voltages have values as shown in the following Table.

    ______________________________________                                         PORVDD              +5 volts                                                   PORVHH              +12 volts                                                  vref                +2.5 volts                                                 vref2               +0.625 volts                                               vp (power-up)       2.45-2.563 volts                                           vp (power-down)     2.536-2.424 volts                                          Rd1                 8.25k ohms                                                 Rd2                 10.0k ohms                                                 Rh1                 31.6k ohms                                                 Rh2                 10.0k ohms                                                 ______________________________________                                          Notes:                                                                         1. divider resistors have ±1% variation total for each pair.                2. resistors Rr1 and Rr2 were ratioed 3:1 to generate vref2.             

The parameters specified in the above example are met by ratioing resistors R2 and R1 (see FIG. 1) to a ratio 209:1, resulting in a large area in an integrated circuit. To minimize the area of R2, a T-network topology, as shown in FIG. 1A, was selected. Referring to FIG. 1A, the equivalent resistance Req can be written as:

    Req=Ra+Rc[1+(Ra+R1)/Rb]

resulting in values Ra=50 k ohms, Rb=5 k ohms, Rc=41 k ohms and R1=2.5 k ohms.

Any of a variety of known comparator circuits may be used in this application. In one example of the preferred embodiment, a comparator design (not shown) was based on a folded cascode topology. It is important that the comparator be able to operate at low voltage, for example, down to 2 volts. Also, the switching speed of the comparator will affect the power-on circuit response time.

The logic gates (22,24,38,40) and flipflop (20) are known and standard CMOS cells exist for their implementation. The resistor networks may be implemented, for example, using interdigitated polysilicon to attain matching accuracy.

Power-Up Sequencing of the Power-On Circuit

It is generally preferred that a power-on circuit itself "power-on" initially asserting the reset signal in the invalid state, thereby ensuring a proper initialization of other circuitry only after the power supply voltage(s) are valid and have stabilized. When power is first applied to the present circuit, it operates as follows. Assume that the power supply input voltages PORVDD and PORVHH are ramped upward. The reference voltage vref is derived such that on power-on vref follows PORVDD until it exceeds the steady state value of vref. During power supply ramp up, vsen1 and vsen2 are less than vref (vp=vref) causing vo1 and vo2 to be high (assuming vref has not reached its steady state value). A logic 1 on vo1 or vo2 sets Q high, discharging the delay capacitor 30 through transistor 32. This ensures that any initial bias or stray charge on the capacitor is removed. The logic 1 at Q also sets reset₋₋ h high, indicating an invalid state of the power supplies.

As PORVDD continues to increase vref attains its steady state value. When both the power supplies reach their valid levels, and vsen1, vsen2 exceed vp by a small increment Δv, outputs vo1 and vo2 switch to logic 0 and flipflop 20 is switched to its reset mode, i.e., Q is logic 0. As a result, current source 36 begins to charge capacitor 30. This begins the delay time period.

Note that at this time vo4 (the output of Comparator 4) is high (vp>DELAY). This holds reset₋₋ h high (the invalid state) until the capacitor is charged to vp, regardless of the flipflop state (Q), which now is low (logic 0). At a conclusion of the delay period, i.e. when the capacitor voltage exceeds vp by a small amount, vo4 goes low. Assuming that Q is still low, the reset output signal (reset₋₋ h) is switched to logic 0, the valid state.

Steady-State Operation

At any time that either one or both of the power supplies' voltage falls below its valid voltage level, the corresponding input comparator switches to a logic 1, and that signal propagates through gates 22,24 to set flipflop 20 so that Q goes high. Q high immediately drives OR gate 40 high and hence the reset signal reset₋₋ h immediately switches to logic 1, indicating an invalid power supply voltage. An advantage of the invention is that this indication of an invalid power supply voltage does not await discharge of the delay capacitor. This feature is best illustrated in FIGS. 3A and 3B, described below.

When the power supply voltage returns to a valid level, the input comparators again are low, so gate 22 goes high. However, gate 38 prevents resetting the flipflop, as noted above, until Comparator 3 indicates (vo3 high) that the capacitor has substantially discharged. This set-up ensures that at least the delay period (td) is provided every time the power supply input voltage becomes invalid.

Illustrative Simulation Results

Operation of the power-on circuit 10 is illustrated in the simulation plots of FIGS. 2A through 2E. FIGS. 2A through 2E share a common horizontal scale which represents time. Each horizontal division corresponds to approximately 100 msec. FIG. 2A shows the comparator output signal vo3. FIG. 2B shows the flipflop output signal q. FIG. 2C shows the reset signal reset₋₋ h and the delay capacitor voltage DELAY. FIG. 2D shows the PORVDD supply voltage along with the reference voltage vref. Finally, FIG. 2E shows the second power supply input voltage PORVHH. The vertical scales, representing voltage, are labeled as appropriate. FIGS. 2A through 2D thus employ two volts per vertical division while FIG. 2E is scaled to 5 volts per division.

Initially, the two power supply input voltages ramp up to their nominal values. PORVHH becomes valid almost immediately, and PORVDD becomes so within approximately 10 msec. Shortly thereafter, referring to FIG. 2E, PORVHH exhibits a brief negative spike 50, at around 20 msec. Referring now to FIG. 2C, this spike delays (or restarts) charging the delay capacitor, so the voltage DELAY does not begin to rise until after the spike 50 has subsided. During the period approximately 25 msec to 400 msec, the reset₋₋ h output is held high (invalid), as the DELAY voltage on the capacitor rises, as shown in FIG. 2C.

FIG. 2A shows the vo3 signal, which initially is high, indicating that the delay capacitor is substantially discharged (DELAY<vref2). At approximately 150 msec, DELAY crosses vref2, and vo3 goes low. This prevents the input comparator output signals (not shown) from resetting the flipflop, even though the power supply voltages are now valid. At approximately 430 msec, DELAY crosses vp (around 2.5 volts) so vo4 goes low (not shown). Since q is still low (FIG. 2B), reset₋₋ h now goes low, as shown in FIG. 2C, indicating a valid state of affairs.

Referring now to FIG. 2D, at approximately 600 msec, PORVDD exhibits a brief negative spike 52 during which the voltage falls below its valid level. PORVDD soon recovers to a valid voltage level, albeit somewhat lower than its initial level. Spike 52 switches vo1 high, which propagates through flipflop 20 to set Q high, which in turn drives reset₋₋ h high (see FIG. 2C), thereby indicating an invalid state. The delay capacitor is discharged, so DELAY falls to zero. Since PORVDD immediately recovered to a valid level, and since vo3 again is high (capacitor discharged), the flipflop is quickly reset so q goes low, starting to recharge the delay capacitor. The q signal thus exhibits a brief positive spike 46 at this time.

FIGS. 3A and 3B are zoom plots, expanded about the 600 msec point, to show detail of the reset₋₋ h, DELAY and vo3 signals (FIG. 3A) and the q signal (FIG. 3B). In response to the glitch on PORVDD at 600 msec (FIG. 2D), reset₋₋ h immediately goes high (invalid). The capacitor begins to discharge so DELAY decays. After about 2 msec, DELAY crosses vref2, so vo3 goes high. Referring to FIG. 3B, q goes high immediately in response to the input glitch. Even though the power supply PORVDD recovered to a valid voltage level immediately, q is held high because vo3 is low, until the capacitor has substantially discharged. Then vo3 goes high, allowing the flipflop to reset, so q goes low. At this time, after 602 msec, recharging begins.

Referring again to FIG. 2E, the PORVHH supply voltage exhibits a second negative spike 54, this time at about 690 msec. Spike 54 includes an excursion below the PORVHH valid level. Accordingly, spike 54 switches vo2 high, once again setting the flipflop (q goes high), and discharging the capacitor (see FIGS. 2B and 2C). As soon as PORVHH becomes valid, since vo3 is high, q is reset (to logic 1) and the capacitor begins to recharge again. The reset output signal reset₋₋ h remains high (invalid) throughout this time, held in that state by vo4 as the capacitor voltage DELAY remains below vp. As shown in FIG. 2C, recharging continues for the remainder of the plot. Reset₋₋ h remains high (invalid). The DELAY voltage ramps up at a lower slope than it did initially, due to the dependence of current source 36 (FIG. 1) on the PORVDD supply voltage. Finally, referring to FIG. 2A, vo3 goes low at approximately 1300 msec, as DELAY exceeds vref2.

Having illustrated and described the principles of our invention in a preferred embodiment thereof, it should be readily apparent to those skilled in the art that the invention can be modified in arrangement and detail without departing from such principles. We claim all modifications coming within the spirit and scope of the accompanying claims. 

What is claimed is:
 1. A power-on circuit for generating a binary reset output signal having a valid state and an invalid state, comprising:first input means for receiving and scaling a first power supply voltage to be monitored; threshold voltage means for providing a threshold voltage; first comparator means for comparing the scaled first power supply voltage to the threshold voltage and indicating a valid power supply state when the first power supply voltage exceeds the threshold voltage and for indicating an invalid power supply state otherwise; output means for providing a binary reset output signal, the output means asserts the binary reset output signal in a valid state responsive to said indication of a valid power supply state and to assert the reset signal in an invalid state responsive to said indication of an invalid power supply state; a delay timer for providing a predetermined delay period and indicating completion of the delay period; means for starting the delay timer responsive to the indication of a valid power supply state; means for delaying said starting the delay timer until after the delay timer indicates completion of the delay period, to ensure that at least the delay period elapses since the first power supply last becoming invalid before changing the reset output signal to the valid state; and means coupled to the first comparator means for forcing the reset output signal to an invalid state responsive to the indication of an invalid power supply state, said forcing means being connected directly to the output means so as to circumvent the delay timer thereby substantially immediately forcing the reset signal to the invalid state without regard to completion of the delay period.
 2. A power-on circuit according to claim 1 further comprising:means coupled to the threshold voltage means for providing hysteresis; a second input means for receiving and scaling a second power supply voltage to be monitored; second comparator means for comparing the scaled second power supply voltage to the threshold voltage and indicating a valid power supply state when the scaled second power supply voltage exceeds the threshold voltage and for indicating an invalid power supply state otherwise; and logic means coupled to the first and third comparator means for indicating an invalid power supply state whenever either one or both of the first and third comparator means indicates an invalid power supply voltage whereby the threshold voltage exhibits hysterisis so that neither the second nor third comparators individually require hysteresis.
 3. A power-on circuit according to claim 2 wherein:the first comparator means includes a voltage comparator having a first input coupled to the first power supply voltage; the third comparator means includes a voltage comparator having a first input coupled to the second power supply voltage; the first and third comparators each has a respective second input coupled to receive the threshold voltage; and the logic means comprises a logic gate having an input coupled to an output of each of the voltage comparators, for combining the voltage comparator outputs so that the logic gate output changes state whenever either one or both of the monitored power supply voltages falls below its respective valid voltage level.
 4. A power-on circuit according to claim 1 wherein the delay timer includes:means for storing an intermediate logic signal; a delay capacitor for storing a capacitor voltage; means responsive to the storage means for charging the delay capacitor while the intermediate signal is in a first state and for discharging the delay capacitor while the intermediate signal is in a second state; second comparator means for comparing the delay capacitor voltage to a predetermined reference voltage and for indicating when the delay capacitor voltage falls below the reference voltage, thereby indicating that the delay capacitor is substantially discharged; and wherein said delay timer starting means is coupled to the first comparator means for setting the intermediate logic signal to the first state to begin charging the delay capacitor responsive to the indication of a valid power supply state, said delay period being approximately the time necessary to charge the delay capacitor; and said means for delaying said starting the delay timer includes the second comparator means and is coupled to the first comparator means for setting the intermediate logic signal to the second state to begin discharging the delay capacitor only when the power supply voltage is invalid and after the delay capacitor is substantially discharged, thereby ensuring completion of the delay period before starting a new delay period.
 5. A power-on circuit for generating a binary reset output signal having a valid state and an invalid state, comprising:input means for receiving and scaling a power supply voltage to be monitored; threshold voltage means for providing a threshold voltage; first comparator means for comparing the scaled power supply voltage to the threshold voltage and indicating a valid power supply state when the power supply voltage exceeds the threshold voltage and for indicating an invalid power supply state otherwise; output means for providing a binary reset output signal, the output means arranged to assert the reset signal in a valid state responsive to said indication of a valid power supply state and to assert the reset signal in an invalid state responsive to said indication of an invalid power supply state; a delay timer for providing a predetermined delay period and indicating completion of the delay period, the delay timer including: means for storing an intermediate logic signal; a delay capacitor for storing a capacitor voltage; means responsive to the storage means for charging the delay capacitor while the intermediate signal is in a first state and for discharging the delay capacitor while the intermediate signal is in a second state; and second comparator means for comparing the delay capacitor voltage to a predetermined reference voltage and for indicating when the delay capacitor voltage falls below the reference voltage, thereby indicating completion of the delay period; means for starting the delay timer responsive to the indication of a valid power supply state; said delay timer starting means being coupled to the first comparator means for setting the intermediate logic signal to the first state to begin charging the delay capacitor responsive to the indication of a valid power supply state, said delay period being the time necessary to charge the delay capacitor from a first voltage to a second voltage, said second voltage having a magnitude greater than said first voltage; means for delaying said starting the delay timer until after the delay timer indicates completion of the delay period, to ensure that at least the delay period elapses since the power supply last becoming invalid before changing the reset output signal to the valid state, said delaying means including the second comparator means and being coupled to the first comparator means for setting the intermediate logic signal to the second state to begin discharging the delay capacitor only when the power supply voltage is invalid and after the delay capacitor is substantially discharged, thereby ensuring completion of the delay period before starting a new delay period; and means coupled to the first comparator means and coupled to the storing means for forcing the reset output signal to an invalid state responsive to the indication of an invalid power supply state, said forcing means including means for connecting the storing means directly to the output means so as to circumvent the delay timer thereby substantially immediately forcing the reset signal to the invalid state without regard to completion of the delay period.
 6. A power-on circuit according to claim 5 wherein:the first comparator means includes a first voltage comparator having an inverting input coupled to the scaled power supply voltage and a non-inverting input coupled to receive the threshold voltage; the storing means includes an RS flipflop circuit; the output of the first voltage comparator is coupled to the set input of the flipflop for setting the flipflop responsive to an invalid power supply state; the intermediate logic signal is the Q output signal of the flipflop; the second comparator means includes a second voltage comparator having an inverting input coupled to the delay capacitor and a non-inverting input coupled to receive a predetermined reference voltage; and further comprising: an AND logic gate coupled to receive the output of the first voltage comparator and the output of the second voltage comparator and having an output coupled to the reset input of the flipflop, for resetting the flipflop to begin charging the delay capacitor responsive to a valid state of the power supply only when the delay capacitor is substantially discharged.
 7. A power-on circuit according to claim 6 wherein:the power-on circuit is implemented in a CMOS integrated circuit and the means for charging and discharging the delay capacitor include complementary transistors each having a gate coupled to the Q output of the RS flipflop circuit.
 8. A power-on circuit according to claim 5 wherein the threshold voltage means includes:a resistive divider network for dividing a predetermined reference voltage; and feedback means coupled between the threshold voltage node and the reset output signal for changing the threshold voltage in response to a state of the reset output signal. 